Are you able to see the IP settings for the FV box when connected (or trying) to the secondary - check that the IP address, subnet mask and default gateway are correct for the secondary router, not the primary.
Does the freeview box have a fixed IP address or DHCP? One plausible answer is that the FV box has a fixed IP address that works with the IP subnet provided by the primary modem/wireless router, but not the second, which is operating with a different IP subnet to the primary. Of course, that assumes that the uplink (internet) port of the secondary is plugged into one of the downlink ports on the primary.
If wireless, the FV box might not have the correct credentials for the secondary wifi network. Maybe the secondary has a MAC address filter set that blocks the FV box. Another possibility is that both routers are providing DHCP addresses in the same IP address range, and a duplicate IP address is on the network.